Output coupling circuit for cavity resonators



Aug. 4, 959 P'. w. CRAPUCHET-TES 2,393,560

OUTPUT COUPLING CIRCUIT FOR CAVITY RESONATORS Original Filed' Nov. 29, 1952 INVENTOR 1? w. CRAPUCHETTE'J p A ORNEY United States Patent OUTPUT COUPLING CIRCUIT FOR CAVITY RESONATORS Paul Wythe Crapuchettes, Atherton, Calif., assignor to Litton Industries, Inc., San Carlos, Calif., a corporation of California Continuation of application Serial No. 323,268, Novem- 'ber 29, 1952. This application December 20, 1957, Serial No. 704,203

5 Claims. (Cl. 333-33 This invention relates to output coupling circuits for electron discharge devices having cavity resonators, and particularly to coupling circuits employing wave guides for coupling the power out of a cavity resonator, such as a resonator cavity in a magnetron oscillator. This application is a continuation of US. Patent application Serial No. 323,268, filed November 29, 1952, now abandoned.

The generation of microwave power at comparatively high electron efficiency is one of the inherent advantages of the magnetron. However, high efficiency may be ob tained only if the coupling to the load is adjusted to an optimum value for all frequencies over the operating frequency band. The search for a broad-band system suitable for coupling magnetron generators to an output system led to consideration of wave guide coupling. Wave guides including special elements were tried, e.g., a quarter wave length section of geometric mean impedance (H section, or ridge wave guide) was employed between the output cavity of the magnetron and the wave guide. However, when the magnetron was tuned over a range of more than :5%, this method was not completely satisfactory. New systems were designed which employed ramps whose impedance varied exponentially, per guide Wave length, from the output cavity load impedance to the wave guide impedance. At higher frequencies the load impedance requires such small spacing between the ramp sections as to render this system impractical. Thus, a system was sought which would permit the use of larger spacing between the ramp sections (when the ramps are used) and at the same time present the frequency vs. impedance characteristic required by the magnetron to operate at high efiiciency.

Accordingly, it is an object of this invention to provide a coupling circuit for a cavity resonator which permits relatively larger spacing between the ramp sections and presents the frequency vs. impedance characteristic required by the magnetron.

It is a further object of this invention to provide a unique broad-band system suitable for coupling a magnetron generator to an output system, by providing cut-off wave guide sections.

It is a feature of this invention to provide a novel means for providing the desired impedance over a given frequency band in a cut-off wave guide section.

In accordance with one aspect of the invention there is provided a coupling circuit interconnecting two sections of a transmission system having different characteristic impedances. The coupling circuit comprises a wave guide equipped with impedance matching means at either ends thereof to match the impedance of the respective sections of the transmission system. The wave guide is so dimensioned that the normal frequency of the energy transmitted through said system is less than the cut-off frequency of the wave guide. The transfer of real energy through the wave guide is effected by the reflection pro- "ice duced by the different impedances at the ends of the wave guide.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 illustrates a cross-sectional view through a multicavity magnetron anode and the output wave guide coupler illustrating the construction of the output coupler in accordance with my invention;

Fig. 2 is a perspective view showing in particular a feature of my invention;

Fig. 3 is a frequency-impedance curve of a cut-off wave guide; and

Fig. 4 is a graph computed from experimental data showing the efficiency using my invention as compared with the prior art.

Although the description of my invention will be made in connection with the magnetron, it is to be understood that the principle of the invention, may be utilized, with equal facility, with cavity resonators in general.

Referring to Fig. 1, there is illustrated a magnetron having an exciting cathode 1 disposed centrally of a cylindrical anode 2. A plurality of radially disposed vanes 3 are fastened at one end to the anode wall 2 to provide successive adjacent cavity resonators. Coupled to the magnetron is an output wave guide 4 comprising a conductive outer wall member 5 which may be cylindrical or any other shape and sealed at one end 6 to the wall section 2. For the sake of simplicity, only the parts necessary to an understanding of the invention are illustrated. For example, the cathode leads and the cylindrical section usually surrounding the anode wall are not shown. The magnetron resonator and the output transmission system are coupled by means of an opening such as slit 7 made in the wall 2. This slit 7 is longitudinally disposed in the cylindrical wall 2 at a radial or non-radial angle, depending on the design of the magnetron. Impedance transformer ramps 8 may be employed to control the impedance of the wave guide with respect to the impedance of the magnetron oscillator as described in my Patent No. 2,555,349, filed August 18, 1948 and issued on June 5, 1951.

The structure described above defines. a known type coupling circuit for a magnetron. Further details of this type system may be found in the above-identified patent. The efliciency of a generator coupled through such a system tuned over a band of frequencies follows a curve such as shown at A in Fig. 4. This curve is for ramp section separated by .003 gap at their convergent ends, and it is seen that the efiiciency diminishes materially from 1700 mc./s. to 2400 mc./s.

In accordance with the present invention, the output transmission system of the magnetron further includes a waveguide section 9 which is dimensional below cutoff at the operating frequency of the magnetron. It should be noted here that the cut-off waveguide section 9, as shown in Figs. 1 and 2, is similar to a conventional waveguide iris, the distinction between an iris and a section of cut-off waveguide being that the latter has a finite length in the direction of propagation whereas an iris has no length for practical purposes.

Consider now the function and purpose of the cut-off waveguide section as it relates to the operation of the magnetron. It is known that the attenuation factor in the propagation constant of a cut-off waveguide is real rather than imaginary as it is in a conducting waveguide, the frequency versus impedance curve for a cut-off waveguide being shown in Fig. 3 from which it may be seen thatthe impedance of the waveguide decreases as the frequency approachescut-oif level. It is also known'that the nature of magnetron operation is such that a fixed electrode geometry 'tends to require a load impedance which varies approximately as frequencysquared.

The characteristic curve of the cut-01f waveguide-most nearly fills this requirement. Thus-to attenuate-thewave in a given frequency range, as for example, between points B and C, the irisring or cut-off waveguide is provided, which may be positioned either adjacent the magnetrons anode-wall, as shown in Fig. 1, or may be further removed from the-magnetron sandwiched between portions of the ramp sections 8. The openingin-the cut off waveguide 9 is. chosen to formthe proper impedance in the waveguide whereby the magnetron efiiciency-curve is in the position-substantially as shown-at' D, in Fig; 4. The attenuation in the output transmission system is thus determined by the dimensionsof the cut-off waveguide, together-With the terminating impedance which is shown illustratively in Fig. l by the bloclcdesignated 10. It should be noted at-this point that the-cross-sectional dimension of the cut-off waveguide will determine its cut-off frequency and its attenuation per'unit length; accordinglyit isapparent that the attenuation ofiacut-off guide and its variation with frequency may be'controlled by control-ling its crosssectional dimen'sionand-length.

Witlrreference now to Fig. 2 it will be noted that-the convergent ends of ramps 8 and the associated conducting output waveguide maybe utilized toprovide a reflective termination at the output'endof the cut-elf waveguide 9 to thereby reflect a portion of the wave transmitted therethrough. It should alsobe noted atthis point. that inputtothe-cut-otf waveguide from the'cavity resonator, as shown in Fig. l by'the slit 7, may be employed to provide an additional matching susceptance for presenting the optimum output impedance tothe magnetrons resonant system.

In constructing an output structure in accordance with the present invention, the impedance'is known which the output cavity resonator likes to see-if the most-power is to be extracted from the magnetron. In addition, an output transformer system can be designed, based'upon dimensionalcontrol limitations or. any other selected criteria, for transmitting energy beyond the cut-off waveguide. The conductance and susceptance ofthe chosen output transformer system may then be calculated, after whichthe desireddecouplingto be provided by't-he cut-off waveguide may be determined, the decoupling being the ratio of the desired.sendingendconductance'to-the-load or; output conductance. One-thenselects thelength and cross-sectional dimensionsfor the cut-off waveguide to provide a value of 61 which meets the broadband'requirements of the output system, keeping in mind that to obtaina change in decoupling with a change in frequency it is necessary to consider the affect of frequency on the propagation constant 8. The selection of the cross-sectional dimension of the cut-off guide-then also defines its surge admittance (yo), and thus will determine what additional matching susceptance should be shunted across the input tothe cut-off guide to providethedesiredimpedance to the cavity resonator.

From the viewpoint of analytic simplicity, it is preferable to use a cut-off section having ramps which extend into the conducting guide since the ramp gap and energy density need not change at the junction, thus reducing discontinuity. From the standpoint of manufacturing ease, however, it is advantageous to construct the guide without ramps, but the problem of discontinuity arises and although solvable, re quires the use of flux plots or emperical measurements to determine the proper dimensionsin any given application.

Although the description of the cut-ofl wave guide output,circuit has been made in connection with a cavity re onatogjheunderlying principle of the invention has much vention in connection with specific apparatus, it is tobe clearly understood that this description is made only by way of example and not as a limitation of thescope of my invention asset forth in the objects thereof andinthe accompanying claims.

What is claimed as new is:

1. An output coupling circuit for a multi-cavity resonator magnetron wherein the magnetron resonators are formed by a plurality of radially disposed vanes connected to an outer cylindrical wall, said output coupling comprising an output structure including longitudinal conductive walls sealed at one end to said cylindrical outer wall, said magnetron wall having a communicating slit between one of said resonators and the interior of said wave guide; a pair ofoppositely disposed ramps affixed-to saidlongitudinal conductive wall to provide a conductingoutput waveguide; and an apertured conductive block positioned in said output structure and forming a com municating'openingbetween saidramps and said slit, the aperture in said block forming awave guide dimensionedbelow cut-off at the resonant frequency of said cavity resonators to provide a frequency sensitive decoupling impedance betweensaid cavity resonators and said con- 7 ducting output waveguide.

2. A broadband output structure for extracting energy from a tunable multi-cavity resonator magnetron, said output structure comprising: a section of-waveguideof predetermined length and internal cross-sectional dimensionspositionedadjacent one of said cavity resonators, said waveguide having a cut-off frequency higher than the resonant frequency of the cavity resonators; input means for transmitting electrical energy from said one cavity resonator to one end of said cut-off waveguide; and output means forming a reflective terminationat the other end of'said cut-off waveguide to permit energy to propogate through said cut-oh" waveguide, said output means including a section of ridged conducting waveguide in which the ridges are formedby a pair of tapered ramp members disposed on opposite sides of said conducting waveguide, the convergent end of said ramps being contiguous with said other end of said cut-elf waveguide.

3. The broadband output structure definedin claim '2 wherein said-ramps are spaced from each other at their convergent end by a distance smaller than the internal dimension of said cut-off waveguide in the plane of said ramps.

4-. In a microwave oscillator including a tunable cavity resonator having an aperture for extracting energy there; from, an energy transfer circuit to permit highly eflig cientopera'tion of said resonator over its frejquency range by presenting a frequency dependent output impedance, said energy transfer circuit comprising: a'waveguide connected at one end to said resonator and positioned to receive energy, through said aperture, said waveguide being cross-sectionally dimensioned below cut-off over the frequency range of said resonator; a conducting output waveguide connected to the other end of said cut; off waveguide; and means forming a non-dissipative reflecting' transition between said cut-off waveguide and said output waveguide, the length andcross-sectionaldimensions of said cut-off waveguide being selected'to provide anoutput impedance to the cavity resonator of the desired magnitude and frequency dependence.

5 ..I,Il av microwavevacuum tube for presenting to an outpupwavjeguidehaving a relatively lughimpedance crowave energy generated in a tunable source of micro wave oscillations whose impedance is relatively low and varies approximately as a function of frequency squared, an output coupling circuit comprising: a section of waveguide dimensioned below cut-01f over the operating frequency range of the microwave source, said waveguide section being connected at one end to the microwave source and at the other end to the output waveguide; and means forming a non-dissipative reflecting transition between said other end of said waveguide sec- 10 2,407,267

5 the operating frequency range of the source.

References Cited in the file of this patent UNITED STATES PATENTS Ginzton Sept. 10, 1946 

